1. Field of the Invention
This invention relates generally to material dispensing nozzles, and more particularly to a method and nozzle assembly for dispensing a viscous material, such as an adhesive, onto a moving substrate in a precise predetermined pattern and with a predetermined frequency, which eliminates any residual material from being dispensed, which can be readily interchanged with another nozzle assembly having a different pattern and, if a hot melt adhesive is utilized, which provides controlled heating of the adhesive throughout the nozzle assembly.
2. Description of the Related Art
Nozzle assemblies are frequently utilized to dispense a viscous adhesive onto containers, such as end flaps of cardboard boxes, prior to closing and filling the container with a desired material. These nozzle assemblies typically include a material flow path through the nozzle which extends between a material reservoir or supply line to a nozzle outlet or dispensing tip and are actuated by a mechanism to selectively dispense the adhesive with a predetermined frequency.
An example of such a nozzle assembly is illustrated in U.S. Pat. No. 4,844,004 which discloses a method and apparatus for applying narrow, closely spaced beads of viscous liquid, such as a hot melt adhesive, to a substrate. The apparatus includes a manifold block in operable communication with a plurality of dispensing guns and a nozzle assembly formed by a shim defining vertical slots therethrough. In order to obtain sharp cutoff of liquid flow, the shim must be sufficiently thin, the width of the slot must be sufficiently narrow, and the length of the slot must be sufficiently long such that the flow resistance is sufficient to cut off the flow of liquid from the exit orifices when the dispensing guns are cut off. Such a structure, however, is extremely complex and requires a multiplicity of components which increases costs and changeover time, thereby reducing flexibility. Additionally, this structure relies on a baffle or restrictor type distribution of the liquid requiring precise tolerances and very small dimensions which are difficult to obtain and maintain. Furthermore, despite the allegation of material cutoff, this structure frequently exhibits "stringing" of material, especially with high viscosity adhesives.
In order to provide container sealing for the packaging of granular products such as rice or powders such as soap, dried milk, sugar or the like, a "siftproof" or "infestation resistant" seal must be provided, especially if a separate liner is not utilized. This type of seal requires accurate positioning and flow of adhesive to eliminate any openings and is typically is considered acceptable if the sealed carton can hold water without leaking.
An example of such a siftproof seal is illustrated in U.S. Pat. No. 5,024,709 which discloses a contact-free method of forming sift-proof seals utilizing an adhesive strip pattern for closure of a container having a plurality of folded flaps. Adhesive dispensing is accomplished by a plurality of vertical nozzles, one of which is substantially is illustrated in FIG. 6 of the present application.
Those nozzles, however, do not provide precise cutoff of the adhesive thereby causing excess accumulation of the adhesive about the nozzle outlets and dripping and stringing of the adhesive into undesired locations on the container. Additionally, since the adhesive is typically heated, the nozzles are positioned at a distance from the heater member which requires the entire head to be overheated in order to maintain the desired temperature of the adhesive. Any overheating of the adhesive can cause degradation of the adhesive and long term contamination. Additionally, such a nozzle is limited by the viscosity of the adhesive which is typically approximately 1,000 centipoise (cp.), but is expected to increase in the future, especially with adhesives for use with coated cartons.
It therefore would be desirable to provide a method and nozzle assembly for dispensing a viscous adhesive, especially a heated viscous adhesive, onto a moving substrate in a precise predetermined pattern and with a predetermined frequency which eliminates any residual adhesive from being dispensed as well as any dripping or stringing of the adhesive, which can readily be interchanged with another nozzle assembly having a different pattern, size or both, and which provides a heater and sensor therein for precise and accurate heating of the adhesive throughout the nozzle assembly.